Blackberry has cornered the military market but after the Department of Defense approved the use of Apple devices, this will likely change.

Blackberry has had a rough go of it for the past couple of years but a change announced last week might be the biggest blow yet. The U.S. Department of Defense (DOD) announced the approval of the use of several Apple devices running iOS 6, including the ever-popular iPhone as well as the iPad and other Apple products.

The move came after the approval of the Apple iOS 6 Security Technical Implementation Guide (STIG), which means that approved, government-issued devices running iOS6 can connect to the secure DOD network. The Defense Department’s Defense Information Systems Agency (DISA) created a Mobility Device Management (MDM) system to support multiple mobility devices on its network.

“All of these pieces must be in place to allow the secure use of commercial mobile devices on department networks,” said Mark Orndorff, DISA information assurance executive & program executive officer for Mission Assurance and Network Ops. “DISA is running a pilot program today where we bring this all together."

While the potential loss of several hundred thousand subscriptions won’t send Blackberry spiraling into bankruptcy, the effect may be more harmful to Blackberry’s reputation.

Kyle Stock explains in his Bloomberg Businessweek article, “Sure, a few hundred thousand here, a few hundred thousand there, and eventually you’re talking real money. But the bigger damage to BlackBerry may be to its reputation for a unique level of network security. Historically, it has wooed corporate and government buyers and slowed defections with the promise of the most hack-proof network on the market. That fight has already been largely lost at the corporate level; when the Department of Defense is cool with the competition, it’s hard for an IT guy to argue.”

DOD’s approval of the Apple iPhone comes on the heels of the department’s announcement that Samsung phones running Google Android can now access secure DOD networks. Blackberry’s Defense Department market share is going to shrink in the coming months, but the big question is, how much will it shrink?